The invention relates to a magnetic coil arrangement of a magnetic field applicator for treating biological tissue, and more particularly a magnetic oil arrangement for heating magnetic or magnetizable substances or solids in biological tissue.
Cancer diseases can be treated in a generally known manner by means of hyperthermia processes, wherein cancer tissue is specifically heated to temperatures of approximately 41xc2x0 C. to 46xc2x0 C. for irreversible damage. In a known hyperthermia process (WO 97/43005) for tumor therapy, magnetizable microcapsules are used which reach the area of the tumor through the blood stream. During a treatment, these microcapsules are charged with a magnetic alternating field generated outside of a patient, with hysteresis effects generating heat for hyperthermia in the microcapsules. A linear magnetic alternating field is used with a frequency in the range of 10 kHz to 500 kHz. The microcapsules should contain a highly magnetizable material so that the force of the magnetic alternating field, the required instrumentation structure, the required cooling system as well as the electrical energy supply can be manageable. A practical instrumentation structure is however not indicated.
In a very much similar, known hyperthermia process (EP 0 913 167 A2), rotating magnetic fields with a frequency in the range greater than 10 kHz are used as fields. To produce the rotating magnetic alternating fields a magnetic field applicator is indicated only sketchily and schematically.
A generic magnetic coil arrangement is shown in the (post-published) DE 199 37 492 publication. The magnetic field applicator for heating magnetic or magnetizable substances in biological tissue comprises a coolable magnetic yoke with two pole shoes facing each other and being separated by a gap to define an exposure volume on the magnetic yoke. To produce a magnetic alternating field, two magnetic coils are assigned to one pole shoe each. The magnetic coils are designed as disk coils with helicoidally extending coil windings and annularly surrounding the pole shoe end of the assigned pole shoes with an intermediate, circulating magnetic coil/pole shoe gap. The magnetic yoke and the pole shoes consist of ferrite block segments which are mounted together.
For hyperthermia, in particular with magnetic liquids, alternating field forces of approximately 15 to 20 kA/m at approximately 50 to 100 kHz are required. With a volume exposed by a magnetic field of 8 to 30 I, effective power of approximatelyl 18 kW to 80 kW must be produced by a hyperthermia installation. This energy must be produced in form of high frequency and must then be transmitted in form of heat with cooling since only a few watts are produced in the magnetic fluid for the hyperthermia in a patient""s body. For cooling of the ferrite block segments, the magnetic yoke and the pole shoes, measures are specified with cooling air flow in cooling gaps. In contrast, the type of cooling of the magnetic coils as well as their mounting system is left open. However, cooling of the magnetic coils is problematic since there is a particularly high power loss which is higher per volume unit than the power loss in the ferrite block segments and since only a relatively small specified space for installation in the magnetic coil area is available for cooling devices and mounting systems.
It is therefore the object of the present invention to develop an improved magnetic coil arrangement for a magnetic field applicator to heat magnetic and magnetizable substances or solids in biological tissue so that effective cooling of the magnetic coils will be possible in combination with a compact arrangement and mounting.
The above objective is accomplished according to the present invention by providing a magnetic coil in a coil box annularly surrounding the assigned pole shoe. The coil box comprises at least one cooling air admission port for connection to a cooling air pump and at least one cooling air discharge port. Magnetic yoke cooling and magnetic coil cooling can be advantageously isolated and optimally adjusted to the different cooling requirements in terms of cooling air volume, cooling air pressure, and cooling air throughput and cooling air flow. Moreover, the coil box can be used, in addition to its duty as part of the magnetic coil/cooling device, for mechanically mounting the magnetic coil. Thus, an advantageously compact design is provided which is well suited to the confined space conditions of a magnetic field applicator in the area of the gap of the exposure volume and a patient""s body areas. In a preferred embodiment, the magnetic yoke and the pole shoes consist of assembled ferrite block segments. The magnetic yoke is combined of cut-stone-shaped ferrite block segments, the surfaces of which are freed from sintering layers and, if necessary, ground to be plane-parallel. The cut-stone-shaped ferrite block segments consist of ferrite plates lined up in a row, aligned in the magnetic yoke along the magnetic flow. The ferrite plates are separated from each other by an insulation/cooling gap transverse to the magnetic flow through which cooling air for magnetic yoke cooling is conveyed. In the direction of magnetic flow, adjacent ferrite plates are separated only by narrow contact gaps. To form the insulation/cooling gap, plastic separators are inserted between the ferrite plates. The cut-stone-shaped ferrite block segments are formed by bonding together the ferrite plates and the separators. The pole shoes are cylindrically or round, as seen from the top, and have a similar structure of wedge-shaped ferrite block segments which are assembled like pieces of a pie. Between these ferrite block segments, insulation/ cooling gaps are also provided by means of separators for pole shoe cooling.
The power losses caused in the ferrite block segments during operation of a magnetic field applicator are so high that they are dissipated by introduction of cooling air into suitably designed insulation/cooling gaps between the ferrite block segments. It has been shown, however, that a possible combination of the magnetic coil cooling and the magnetic yoke and pole shoe cooling is difficult to design, expensive and ineffective. One problem with the possible combination is the fact that the magnetic coil produces a higher power loss in comparison per volume unit. Thus, especially with the arrangement and isolation of the cooling systems according to the present invention provide considerable benefits regarding the arrangement, dimensioning and operation of the two cooling systems. Moreover, its simple assembly also reduces the expenditures for installation, handling and maintenance as well as operating costs.
According to one aspect of the invention, the pole shoe end surfaces are each covered by a pole shoe plate. A laterally surrounding pole shoe plate extends beyond the assigned pole shoe end surface and forms a coil box bottom wall on the side of the exposure volume. Separators are inserted between the pole shoe end surfaces and the pole shoe plate to create insulation/cooling gaps. These separators are relatively small compared with the contact surface of the wedge-shaped ferrite block segments so that a cooling air flow through the separators passes radially between pole shoe end surface and pole shoe plate will hardly be obstructed. The pole shoe plate, in the area of the pole shoe end surface, has an indentation which is less thick than an adjacent area of the coil box bottom wall. The pole shoe end surface extends some-what into this indentation with the surrounding edge of the pole shoe end surface being rounded off. A surrounding annular gap is created as a cooling air outlet between the pole shoe plate and the pole shoe end surface. In this annular gap, it is possible to bypass the radial cooling air flow to an axial outlet direction. The pole shoe plate may be made of insulating material, such as glass. However, a high-quality, fiberglass reinforced plastic is preferably used, and the afore-mentioned indentation can be made by routing.
In an embodiment which is simple to make and functional in design the pole shoes are circular as seen from the top and the magnetic coils are accordingly designed in form of a circular ring. However, the associated coil boxes should be designed cut-stone-shaped with regard to their outer dimensions and surround the pole shoe ends as well as the magnetic coils placed above. On the one hand, a cut-stone-shaped design of the coil boxes results in simple manufacture since no bent wall parts must be connected with each other. Moreover, a favorable arrangement of cooling air admission ports will result which may be arranged either on the coil box side walls and/or in preferably two opposite corner areas of the coil box top wall. With these technically advantageous air admission designs, the required ports as well as flange connections for cooling hoses to be connected can be made with little expenditure.
In another preferable development, the magnetic coil is provided with a support structure for the windings. In the area of the magnetic coil, web-shaped bottom side coil carriers below web-shaped top side coil carriers are provided as coil carrier pairs which are assigned to each other and which are radially arranged in form of rays and angularly spaced to each other like spokes. The assigned coil carrier pairs are each connected by insulating rods which are radially at a distance. The coil carrier pairs are approximately, axially aligned so that retention sections are formed between the insulating rods in which the helicoidally extending coil windings are taken up and held. Due to the web height of the bottom side coil carriers, the coil windings are raised versus the coil box bottom wall forming a radially exterior annular cooling air inlet gap. Cooling air can then be further axially conveyed through this annular cooling air inlet gap and through the intermediate gap defined by the insulating rods between the coil windings. A top cutout section, preferably a circular top gap between the top side coil carriers, which are not covered above the coil windings, will be used as the cooling air discharge port. The height and length of the coil carriers as well as the insulating rods are to be selected such that, on the one hand, the windings will be sufficiently supported and held and, on the other hand, that the insulation distances between the windings comply with the regulations on air and creepage distances, and that sufficient cooling air can be conveyed between them. Particularly advantageous conditions result according to the invention if the insulating rods are designed as round ceramic rods. A practical support structure tested with good results consists of 16 coil carrier pairs with six insulating rods each with five winding retention sections each resulting thereby. The coil windings are designed of a strand of very fine RF wires. The coil box and the support structure for the magnetic coil can each be manufactured alike for the top and the bottom pole shoe. Since, however, the pole shoe plate in the arrangement on the bottom pole shoe faces toward the top, the top-side coil carriers bear the weight of the assigned magnetic coil.
In an advantageously designed further development according to the invention, the coil box bottom wall, the coil box side walls, the coil box top wall as well as a thin-walled wind box inside wall will form a surrounding wind box with a bottom side annular cooling air inlet gap. Moreover, the magnetic coil is surrounded radially on the inside by an air guide wall so that an annular gap is created for the discharge of the pole shoe cooling air between the air guide wall and an adjacent pole shoe wall. In this case, the pole shoe cooling air and the magnetic coil cooling air are separate from each other in the area of this air guide wall and advantageously isolated. The wind box, as a pressure chamber, can be charged with cooling air which is then preferably conveyed via the bottom side annular cooling air inlet gap to the bottom coil winding areas where maximum heating of the magnetic coil takes place, and subsequently is dissipated between the coil windings. Another important improvement of the magnetic coil cooling results from the coil carriers being designed wedge-shaped so that the radially exterior coil windings, with their bottom side coil winding areas, are raised more from the coil box bottom wall than the coil winding areas which are lying radially further inside. Thus, cooling air will be conveyed through the bottom side annular cooling air inlet gap and impinged on the spaced lower edges of the coil windings, where maximum heating of the coil takes place through eddy currents in the copper due to the generated magnetic field. Due to the wedge-shaped design especially of the bottom side coil carriers and the resulting cross-sectional constriction toward the center, the advantageously high air velocity results on the innermost coil winding, i.e. where there is the maximum need for cooling. Cooling air here flows through the winding spacings and can freely leave above the magnetic coil with no further bottlenecks arising. To further convey the cooling air flow to the radially inner coil winding area, at least one approximately bottom-parallel air baffle plate can be arranged starting from the annular cooling air inlet gap. Advantageously, two air baffle plates, one atop the other, are each provided in the area between two bottom side coil carriers, with the air baffle plate which is closer to the bottom to be designed longer and wider. These air baffle plates can simply be screwed to the coil box bottom wall by means of spacers and/or distance rings.
The coil carriers may be provided with location holes for holding the insulating rods, and the bottom side coil carriers may be screwed and/or bonded with the coil box bottom wall, in particular the pole shoe plate. In contrast, the topside coil carriers are radially screwed to the top wall and are detachable on the outside. On the inside, the coil carriers are detachably screwed via support columns with the coil box bottom wall. The detachability of the topside coil carriers is essential for simple assembly of the coil windings. A solid coil box is created through screw and/or bonding connections between the coil box bottom wall, the coil box side walls and the coil box top wall. The coil box is connectable via additional connecting elements such as for example threaded rods with adjacent magnetic yoke elements. A pole shoe plate is stiffened through its lateral connection with the coil box side walls so that it has advantageously only a slight sag although, if necessary, a routed indentation may be provided in the pole shoe area.
An especially preferable combination of the arrangement in accordance with the invention results from a magnetic yoke form known per se according to claim 13 in the type of an M-shape as a three-legged arrangement.